Phase change technology is promising for next generation memories. It uses chalcogenide semiconductors for storing states. The chalcogenide semiconductors, also called phase change materials, have a crystalline state and an amorphous state. In the crystalline state, the phase change materials have a low resistivity, while in the amorphous state they have a high resistivity. The resistivity ratios of the phase change materials in the amorphous and crystalline states are typically greater than 1000 and thus the phase change memory devices are unlikely to have erroneous reading. The chalcogenide materials are stable at certain temperature ranges in both crystalline and amorphous states and can be switched back and forth between the two states by electric pulses. One type of memory device that uses the principal of phase change in chalcogenide semiconductors is commonly referred to as phase change random access memory (PCRAM).
PCRAM has several operating and engineering advantages, including high speed, low power, non-volatility, high density, and low cost. For example, PCRAM devices are non-volatile and may be written into rapidly, for example, within less than about 50 nanoseconds. The PCRAM cells may have a high density. In addition, PCRAM memory cells are compatible with CMOS logic and can generally be produced at a low cost compared to other types of memory cells.
FIG. 1 illustrates a conventional bottom-contact PCRAM cell. Phase change material 2 is formed between top electrode 4 and bottom electrode contact 6. In a reset operation, phase change material 2 may be heated up to a temperature higher than the melting temperature when a current passes through it. The temperature is then quickly dropped below the crystallization temperature. A portion of the phase change material, as schematically shown in region 8, is changed to an amorphous state with a high resistivity, thus the state of the PCRAM cell is changed to a high-resistance state. Region 8 can be set back to the crystalline state by heating up the phase change material 2 to a temperature higher than the crystallization temperature, but below the melting temperature, for a certain period.
The PCRAM memory cell as shown in FIG. 1 suffers from drawbacks when integrated with the manufacturing processes of logic devices. The PCRAM memory cell requires three or more photo masks in addition to existing logic circuit manufacturing processes. For example, each of phase change material 2 and top electrode 4 requires one photo mask. Bottom electrode 6 and top electrode contact 10 in combination at least need one additional photo mask. In addition, heater 12 may be needed to generate heat for the phase transition, and hence the number of additional photo masks is increased to four. Accordingly, there is the need for reducing the manufacturing cost of the PCRAM by reducing the number of photo masks.